Electronic components and integrated circuit packages are often mounted on printed circuit cards creating an electronic assembly of irregularly shaped packages bonded to a planer surface. Multiple circuit card assemblies may be placed into an array (referred to as a cardcage) within an enclosure, thereby maximizing the packaging volume efficiency. Several types and configurations of enclosures with the ability to support various numbers of circuit cards are readily available for connecting the circuit card assemblies into a cardcage. Standard bus architectures have been defined for these card enclosures. For example, the VME bus standard (ANSI/IEEE Std. 1014-1987) has been defined for the electrical backplane bus interface for circuit card enclosures. Other bus standards include the VME64 Standard, which revises the VME bus standard into a 64-bit architecture, and the VXI ("VME extensions for Instrumentation") bus, which is designed to allow low level signals to co-exist on a backplane with high speed digital and RF or microwave signals.
As circuit cards are being developed with greater electronic component density per card, and electronic components continue to operate at higher frequencies and increased power, the cooling of circuit card assemblies in these electronic enclosures becomes more critical for proper operation of these electronic components. Electronic components that operate above recommended thermal constraints may not operate reliably, causing circuit cards to prematurely fail due to thermal stress. The problems associated with overheating of electronic components may be intensified in applications that require circuit card assemblies to operate in harsh environments, such as heavy industrial or military applications.
Traditionally, the cooling of circuit card assemblies in electronic enclosures has been performed by placing fans or blowers at the entrance or exit of an electronic enclosure and forcing air across the circuit card assemblies within a cardcage. The amount of cooling provided by this method is based directly upon the velocity, temperature, and atmospheric pressure of the air flowing over electronic components. Therefore, it is the responsibility of electronic enclosure designers to meet the cooling requirements specified by electronic component and circuit board manufacturers.
However, generally speaking, the cooling requirements specified by electronic component and circuit board manufacturers are incomplete or improperly applied as discussed below. In addition, there is no consistent method used by manufacturers to define cooling requirements. Two general approaches are to specify (1) the maximum component temperature or (2) the maximum operating environmental conditions. The first approach, however, fails to recognize that all components have internal and surface thermal gradients present. Therefore, when specifying the maximum component temperature, the specific location as to where to measure this temperature must be referenced. This detailed information is not generally provided by device manufacturers and is very difficult to obtain from technical representatives. Similarly, when specifying the maximum operating environmental conditions that a circuit card or component can operate in, the manufacturer should identify several parameters. These parameters include air temperature, air velocity profiles, and atmospheric pressure conditions. Manufacturers will generally define the air temperature and mass flow rate requirements. However, no consideration is given to the airflow profile, or it is assumed to be uniform, which is rarely the case. Many times the maximum component temperature is incorrectly assumed to be the same as the maximum air temperature.
Because the cooling requirements specified by electronic component and circuit board manufacturers are often incomplete, testing the components and circuit boards is the simplest way to determine actual cooling requirements. These tests require techniques that can measure device surface temperatures without interrupting the air velocity profile. In addition, these techniques must include a means to measure the air velocity profiles and correlate this data to the measured device surface temperatures. Finally, the techniques must correlate device surface temperatures and air velocity profiles with inlet air temperatures and atmospheric conditions.
In addition to the specifications defined by component and circuit card manufacturers, electronic enclosure manufacturers specify the cooling performance of an electronic enclosure based on an average volumetric flow of air for a single slot within a cardcage. To meet the cooling requirements as specified by the electronic component manufacturers, electronic enclosure designers have traditionally selected a fan or blower with an overall volumetric flow rate that, when divided by the number of slots in a cardcage, meets the specified average flow rate specified for each circuit card assembly. This average flow rate is assumed to be uniformly distributed across each slot in the cardcage and across each of the various electronic components on each circuit card assembly. For example, when designing an electronic enclosure system with six circuit card assemblies in a standard VME cardcage enclosure, the selected fan must provide an average volumetric airflow six times the required volumetric airflow at the circuit card assemblies. The volumetric airflow requirements are derived from the electronic component manufacturers' airflow velocity requirements at the device. If the electronic components require an airflow velocity of 300 linear feet per minute at the device surface, then an overall volumetric flow rate across the circuit card assembly can be derived based on an estimated cross-sectional area between each circuit card assembly within the array. An estimated cross-sectional area between standard 6U-VME circuit card assemblies is 2.5 square inches (0.4".times.6.0"); therefore, the average volumetric flow rate required across each circuit card assembly is 5 cubic feet per minute. Traditional electronic enclosure manufacturers would select a fan or blower that provides six times the average volumetric flow rate for a single circuit card assembly or 30 cubic feet per minute of total flow for the enclosure.
This design method, however, fails to recognize that fans and blowers do not provide uniform flow rate to all circuit card assemblies, nonetheless uniform airflow to the electronic components on each circuit card assembly within the cardcage. Further, this method fails to recognize that a single value for the volumetric flow rate for each circuit card assembly or each electronic component on a circuit card assembly is not adequate to describe the actual cooling requirements for an entire array of circuit cards within an electronic enclosure or even a single circuit card assembly within the cardcage. In addition, this method fails to recognize that there are pressure drops associated with airflow restrictions in the electronic enclosure system. Board and enclosure designs reduce the airflow delivered by the fans. Therefore, any attempt to provide a more uniform airflow must be done without significantly increasing the flow resistance within the enclosure. Additional pressure drops can quickly reduce the amount of air received, and while the air may be uniform, it may be of such low velocities that is negates any improvement in cooling.
It has been recognized that axial fans, centrifugal blowers, and other airflow sources do not provide uniform airflow rates to all of the slots or components on circuit cards within a cardcage. This problem is compounded when different components on a given circuit card assembly or different locations within the cardcage have different cooling requirements. The problem may further be compounded if some of the slots do not contain circuit card assemblies.
Some cardcages have been designed in an attempt to improve cooling under these circumstances. For example, some VXI cardcages provide a mechanical baffle to close the inlet to unused card slots. Other VME cardcages have used slot "blocker" cards to reduce the airflow through unused slots in the cardcage. Although these designs provide some improvement to the problems associated with meeting cooling requirements, these designs do not attempt to provide uniform airflow to all components and circuit card assemblies in a cardcage. Nor do they attempt to control the airflow directly from an airflow source in order to provide cooling at specific locations of a circuit card assembly in a cardcage.
Therefore, a continuing need exists for controlling airflow in a cardcage such that airflow can be provided at specific locations in the cardcage to ensure that each device receives sufficient airflow. Especially needed is a more uniform flow profile wherein air or another gas flows in a pattern within a cardcage such that a circuit board's device temperatures are independent of the boards location within the cardcage. Uniform airflow is essential to proper and adequate cooling of the cards. Proper and adequate cooling, in turn, is essential to the reliable and effective operation of the cards.